Gas sensors are employed in a variety of applications requiring quantitative and qualitative gaseous determinations. In the automotive industry, it is well known that the oxygen concentration in the automobile exhaust has a direct relationship to the engine air-to-fuel ratio. Oxygen gas sensing devices are commonly employed within the internal combustion control system of the automobile to provide accurate exhaust gas oxygen concentration measurements for determination of optimum combustion conditions, maximization of efficient fuel usage, and management of exhaust emissions.
Typically, the oxygen sensors employed by the automotive industry are either electrochemical-type or resistive-type oxygen sensors. The electrochemical-type oxygen sensors are most common and comprise an ionically conductive solid electrolyte material, typically zirconia stabilized by the addition of yttria, a porous electrode coating on one face of the solid electrolyte contacting the external gas to be measured, and a porous electrode coating on the opposite face of the solid electrolyte contacting a known concentration of reference gas. The gas concentration gradient across the solid electrolyte produces a galvanic potential which is related to the differential of the partial pressures of the gas at the two electrodes. Resistive-type oxygen sensors generally comprise a layer of semiconductor oxide material which contacts the exhaust gas. The exchange of oxygen between the oxide's lattice and the exhaust gas causes the electronic resistivity of the oxide material to vary. This change in electrical resistance is related to the oxygen partial pressure of the exhaust gas.
These above mentioned electrochemical-type and resistive-type oxygen sensors are adequate for current automotive needs, which only require that the oxygen sensor determine qualitatively whether the internal combustion engine is operating at either of two conditions: (1) a fuel rich or (2) a fuel lean condition, as compared to stoichiometry. After equilibration, the exhaust gases from these two operating conditions have two widely different oxygen partial pressures. This information is provided to an air-to-fuel ratio control system, so that it can provide an average stoichiometric air-to-fuel ratio between the two conditions. However, due to the increasing demands for improved fuel utilization and emissions control, it is desirable to operate internal combustion engines exclusively within lean combustion parameters, i.e., air-to-fuel ratios between 15:1 and 25:1, where changes in the after-combustion oxygen partial pressures are comparatively slight and gradual. The galvanic-type and resistive-type oxygen sensors currently in widespread use are not sensitive enough for this operating environment.
I propose making a more sensitive oxygen sensor, that functions by optical detection means. Gas sensors using optical detection means to sense a desired gaseous component within a gas mixture are also known, and are generally used to detect the presence of a pollutant or toxic substance. Typically these prior optical-type gas sensors comprise a surface coated with an indicating material, the indicating material capable of producing a color change when contacted with the desired gaseous component. The color change is related to the concentration of the desired component in the gaseous mixture. A photoelectric device, or other similar means, detects the change in color and generates an electric signal proportionate to the color change and indicative of the gas concentration.
These optical-type gas sensors are useful when sensing the presence of an undesired component. However, they are not suitable for automotive use. They are typically bulky, have relatively slow response times, and are generally capable of only one determination of the desired gaseous component, since the chemical reaction causing the color change is typically not reversible.
To be an effective component of an automobile internal combustion control system operating exclusively within lean combustion conditions, the oxygen sensor must be extremely sensitive and capable of rapid, precise, and continuous (i.e., repetitive) oxygen concentration measurements. It is desirable that the response time of the sensor be less than 0.1 second at a minimum temperature of 300.degree. C. and a maximum oxygen concentration at the sensing medium of about eight percent. Further, it is necessary that the oxygen sensor withstand high temperatures. It is also desired that the oxygen sensor be compact, yet structurally durable to withstand the harsh automotive environment.